GB2208019A

GB2208019A - Method of generating a stabilised low voltage

Info

Publication number: GB2208019A
Application number: GB8818329A
Authority: GB
Inventors: Erwin Potthof
Original assignee: Diehl GmbH and Co
Current assignee: Diehl Verwaltungs Stiftung
Priority date: 1987-08-14
Filing date: 1988-08-02
Publication date: 1989-02-15
Anticipated expiration: 2008-08-02
Also published as: FR2621751A1; GB8818329D0; DE3727117A1; GB2208019B

Description

1 2r r.' A METHOD OF GENERATING A STABILISED LOW VOLTAGE The invention

relates to a method of generating a stabilised direct low voltage from a considerably higher 5 pulsating direct voltage for operating electrical loads. The invention relates furthermore to a power supply apparatus for carrying out the aforesaid methods.

A variety of power supply circuits, which transform 10 an alternating voltage by means of a transformer or a capacitor into a lower voltage and possibly stabilise this, are known. For example, on page 208 of the book "Elektronik/Nachrichtentechnik" (Frankfurter Fachverlag, Kohl & Noltemeier-Verlag 1980) there is disclosed, inter alia, the use of an operational amplifier as a voltage regulator. If several direct voltages are to be generated at the output with very different voltage values, then such circuits are only restrictedly capable of use, since at the transistor in the series arm or line the differential voltage, between the voltages at the output and the input voltage, is dropped and is converted into heat. If such circuits are to be operated at a high ambient temperature, as is for example the case with cooker time switches, in the case of which ambient temperatures up to 120 can occur, then such dissipative circuits are no longer usable.

Far less dissipative than circuits of the aforedescribed type are socalled switching power supply circuits, in which the series transistor no longer works in the analog region, but in switching operation. The losses and thus the temperature transistor are considerably less switching power supply circuit, is indicated page in the third figure technical and cost expenditure in generated at the An appropriate shown in the abovef rom the top. The the case of such a 2 circuit is, periodically rather high.

however, an account of the timer, which switches the series transistor through, The task of the invention is to indicate a method and power supply apparatus with which fairly large pulsating direct voltages can be converted into considerably smaller, stabilised direct voltages with good efficiency and slight constructional expenditure.

In accordance with the invention there is provided a method of generating a stabilised low voltage from a considerably higher pulsating direct voltage, for operating an electrical load, characterised in that an output capacitor is charged in accordance with the energy f low-of f to the load, in that the energy supply to the capacitor is effected by switching of a switching element in the circuit of the capacitor respectively at both flanks of each sine half -wave or like pulse of the- pulsating direct voltage supply close to the zero or low point between successivd pulses, in that the switching element is switched on and off by a comparator arrangement which effects an evaluation, in conjunction with a combination, of voltage variations at the capacitor and voltage variations of the pulsating direct voltage together with a reference voltage.

In this method the capacitor at the output is charged by way of a switch to such an extent as is required to sustain the operating voltage against the demand of the load connected in parallel with the capacitor. The energy supply is ef f ected by operating the switch in such a way that close to the zero passage - at each of the two edgei, namely the rising and falling -flanks, of each pulse or half-wave of the input pulsating direct voltage - the switch is closed brief ly f or the recharging of the capacitor. The actuation of this 1 switch is effected in this respect by evaluation of the.voltage applied to the capacitor, of the voltage varying at the input, that is to say in front of the switching element, as well as of a reference voltage.

In general the invention provides a method of and apparatus for generating a stabilised low voltage output from a higher pulsatory direct voltage supply, for supplying a load from an output capacitor, wherein the capacitor is charged by switching on the supply to the capacitor during periods in which the pulsating voltage is rising f rom and f alling to zero or a minimum, and wherein said switching is controlled by comparative evaluation of the supply voltage, the output voltage and a reference voltage to effect switching on only when both said supply and output voltages are below predetermined limits.

Preferably, the comparison is made between the reference voltage and a direct electrical combination of voltages derived from the supply and output voltages. However, instead the comparison may be made between a voltage derived from the output voltage and an electronic-logic combination of the reference voltage and a voltage derived from the supply voltage; or the comparison may be made in stages firstly between the reference voltage and a voltage derived from the output voltage, and secondly between the result thereof and a voltage derived from the supply voltage, preferably by further logic.

The method in accordance with the invention is all the more favourable, the greater the difference between input and output voltage is. With a view to a favourable efficiency, however, the ratio of the two voltages should be at least 5:1.

4 The present invention further provides apparatus for supplying a stabilised low voltage output from a pulsatory direct voltage supply for carrying out the method comprising a switching element, switching transistor, arranged between one of terminals and one of the output terminals; such as a the input an output capacitor lying in parallel with the output terminals, which capacitor is chargable by the switching element in accordance with its discharge in a load by switching current surges generated from both flanks of each input sine half-wave or like pulse; and a comparator arrangement provided with or providing switching hystersis and a reference voltage (UR), one input of which comparator arrangement receives a voltage (UIA) derived from the voltage at the capacitor; wherein a voltage (UIE) derived from the pulsating direct voltage supply is linked electrically or logically with said one input, the reference volage or an output of the comparator arrangement to give a switch control signal; and wherein the switch control signal output of the comparator arrangement is connected to a control input of the switching element so that the latter switches through or respectively blocks said surges upon reaching of predetermined comparative conditions of said voltages (UR),(U'E) and W'A, The switching element preferably comprises a first transistor which is arranged in series with said ones of the input and output terminals and which is switched by a second transistor arranged in a shunt circuit and controlled by the switch control signal.

The apparatus is preferably arranged so that said surges have a maximum voltage of less than three times and preferably a maximum voltage of about 2.8 times, the predetermined output voltage, which surges are conducted 1 i :1 is via a limiting impedance and the switching element to the output capacitor, so that there is minimal dissipation of said surges.

The invention will be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein:- FIGURE 1 shows a block circuit diagram of the switching power supply arrangement in accordance with the invention; FIGURE 2 shows a diagram of the pulsating direct voltage input and points at which switching occurs; FIGURE 3 shows a diagram of the control voltage at the switching transistor of the arrangement; FIGURE 4 shows diagram of the output current of the switching transistor of the arrangement; FIGURE 5 shows a first exemplified embodiment of the switching power supply arrangement; FIGURE 6 shows a second exemplified embodiment of the switching power supply arrangement.

Referring to the block circuit diagram shown in FIGURE 1, at input terminals 1 and la there is applied a pulsating direct voltage UE, which is generated, for example, by a full- wave rectifier which is not shown. At output terminals 2 and 2a there lies a load 3, which can preferably be the circuitry of an electronic cooker time switch. Arranged in the series line between the terminals 1 and 2 is an electronic switching element 4 as well as a low-impedance limiting resistor 5. The electronic switching element can be designed as a 6 switching transistor or as a thyristor. Arranged in parallel with the load 3, between the terminals 2 and 2a, is a supply capacitor 6 which possesses an adequate storage capacity for short-term operation of the load.

The control of the electronic switching element 4 is effected by a comparator or comparator arrangement 7 which has a switching-hysteresis (an abrupt switch-over output characteristic and not a gradually changing output). The comparator receives the voltage necessary f or its operation through a voltage supply 8. At one input of the comparator there are applied, in direct electrical or logical linkage, the voltages U'E and U'A; the first-mentioned voltage is derived by way of a voltage divider 9 from the input voltage UE, the secondmentioned voltage is derived f rom the output voltage UA at the capacitor 6 and, if necessary, reduced by an actual-value converter 10 to a suitable voltage level. At the second input of the comparator 7 there is applied a reference voltage UR, which is generated by an assembly 11, for example a voltage divider.

Whilst the input voltage UE amounts to 220 V, the output voltage UA is to amount to only 20 V. In order not to convert the voltage difference between UE and UA in the switching element 4 into thermal energy, this is operated in switching operation and with high efficiency. Figures 2 to 4 are utilised to explain the function of this apparatus. As a result of the arrangement of the comparator 7 in the control line of the switching element 4 it is possible, in the case of the depicted circuitry of the inputs of the comparator, to actuate the electronic switch at both of the two f lanks of a sine half-wave or other pulse of the voltage UE adjacent to the zero or minimum voltage point whereby to obtain an energy transfer to the capacitor 6 which can be twice as high as that which can be obtained by conventional phase 7 cutting or working controls which switch only one of the two flanks. In the case of the known phase working controls, in order to make available the same charging energy for the capacitor, switching would have to be effected at a higher switching voltage, i.e. further up the flank, which would however lead to a higher power loss or dissipation in the switching element 4 and at the limiting resistor 5.

In the sense of the invention it is important that in the comparator a voltage derived from the capacitor 6 and a voltage derived from the pulsating input voltage UE can be jointly processed with a reference voltage. As a result of the voltage UlE, provided that this voltage is smaller than UR, the switching instant at each flank of each sine halfwaves is fixed to give a switch-on condition. As a result of the voltage U'A, provided that this voltage is 'likewise smaller than UR during the switch-on condition afforded by the voltage U'E, the condition for switching on the switching element 4 is fulfilled once, or even several times, in the comparator 7, so that this switching element 4 (during the period of the switch-on condition of U'E) is switched on once or several times.

1 Shown in FIGURE 3 is the comparator output signal US for the control of the switching element 4 which signal is illustrated, for a single switching on procedure during the whole period, of the switch-on condition.

Independently of the hysteresis of the comparator 7, and even also the current requirement of the load 3, however, the said switching on of the element 4 can be effected a number of times during said period, by splitting up the signal U into a pulse train of several signals.

8 Important to the apparatus is the fact that the comparator 7 or an arrangement including the comparator has a "switching-hysteresis" i.e. provides an output which is of stepped form or otherwise changes rapidly' between distinct levels so that, for example, the signals US are of square wave form. In this way a uniform switching operation of the switching element 4 and of the charging procedure at the capacitor 6 is possible, and an undefined "pendulating" or wavering of the switching operation, e.g. in a partially conductive manner, about the switching point of the switching element is avoided.

Whereas a preferred method of performance of the invention is shown in FIGURE 1, the principle in accordance with the invention can also be realised in that the voltages U'E and U'A are not directly linked electrically at one input of the comparator, but that the voltage WE is linked with the voltage UR at the inputs of an AND-NOT gate of the comparator arrangement, which gate lies at the second' input of the comparator. Also, it is possible, within the scope of the invention, to link the voltage U'E not at the input of the comparator 7, but at an OR-gate, of the comparator arrangement, with the output of the comparator 7 to provide the switch control signal US. However, all such variations also serve to effect a linkage of two voltages and a comparison in the comparator.

The comparator 7 - may comprise an operational amplifier or a Schmitt trigger. In the case of the operational amplifier measures have to be taken in the circuitry to generate a switching- hysteresis. A further possible form of comparator is an OR-gate with two defined switching thresholds, high and low, at its two inputs. The threshold voltages or one thereof may constitute the reference voltage.

I 1 1 9 A first exemplified embodiment of the invention is shown in detail in FIGURE 5. Lying at the input of the circuit is a full-wave rectifier 20, which applies a pulsating direct voltage to the input terminals 21 and 21a. At the output terminals 22 and 22a there lies an electrical load 23. The switching element in the main line between terminals 21 and 22 is in the f orm of an electronic transistor switch comprising two transistors 24 and 24a in Darlington circuit. A limiting resistor 25 is arranged in series in the main line of the circuit, which resistor could, in the appropriate circumstances, be replaced by a small choke. In parallel with the load there lies at the output between the terminals 22 and 22a a capacitor 26. Serving as the comparator is an operational amplifier 27 which receives its operating voltage by way of a resistor 28 and a sub-circuit 35 which includes a Zener diode, a resistor and a smoothing capacitor. The voltage UIE is generated at a voltage divider consisting of a resistor 29 and a resistor 30.

The voltage U'A arises at a divider consisting of a Zener diode 31 and a resistor 32. This voltage is linked electrically by way of a diode 33 with the voltage U'E at the input 27a of the operational amplifier. At the input 27a there lies furthermore a resistor 34, which is connected to the output of the operational amplifier 27 and serves to generate a switching-hysteresis. From the operating voltage (at the circuit point 36) of the operational amplifier 27 a reference voltage is derived, by way of a resistor 37 and a Zener diode 38, which voltage is applied to the second input 27b of the operational amplifier. Lying in parallel with the Zener diode 38 is a capacitor 39 which ensures that the reference voltage is applied only an instant later than the operating voltage potential at point 36 to the operational amplifier.

Connected subsequent to the operational amplifier 27 by way of a resistor 40 is a switching transistor 41, which transistor 41 lies in series with the transistors 24 and 24a in a transverse branch of the circuits andwhich is connected to the base of the transistor 24a. At the base of this transistor 24a there lies furthermore a resistor 42 for the supply of the base current from the main line.

The mode of operation of the described arrangement is now as follows. At the instant of the switching-on of the apparatus let it be assumed that the capacitor 26 is discharged. Its voltages UA and U'A are nil. The reference voltage UR is determined by the rating of the component parts at 5.1 V. By reason of the ratio of the resistors 30 and 29, e.g. of 10:1, there is formed in the case of a voltage of UE of. 56 V, a voltage of 5.1 V at the input 27a of the operational amplifier. As soon as, during the sine half-wave, this value UE of the pulsating direct voltage is fallen below, the output signal of the operational amplifier becomes low and the transistor 41 is blocked. This leads in turn to the switching of the transistors 24a and 24 to a conductive state, thus closing the switch so that a charging current limited only by the resistor 25 flows into the capacitor 26. This charging current, which has a course in accordance with FIGURE 4, persists until, after the zero passage, in the course of the next sine half-wave again the value 56 V for UE is reached. At this moment the output signal of the operational amplifier 27 switches to high and the transistor 41 becomes conductive in series with the resistor 42 so that the base potential of the transistors 24a and 24 drops and both transistors are blocked i.e. switched to being non-conductive to open the switch. The charging of the capacitor 26 ceases. This procedure is repeated during several sine half-waves of the pulsating direct voltage until the capacitor 26 has reached its i i operating voltage. The load 23 is operated by the discharge-current of the capacitor 26. By reason of the size of the capacity of the capacitor 26, the voltage thereof during two sine half-waves drops only immaterially. At the resistor 32 a voltage associated with the voltage at the capacitor is taken off and passed as voltage UIA to the input 27a along with linkage with the voltage U'E. If the capacitor 26 is fully charged, the voltage U'A - UDiode (the voltage drop across diode 33) is greater than UR and even upon existence of a switch-through condition of the voltage U'E - namely UE< 56 V - the voltage at the input 27a is greater than 5.1 V and thus the output signal of the operational amplifier becomes high. Therefore also upon falling below of a voltage of 56 V by UE the transistor 24 is not switched through to be conductive and the capacitor 26 is not recharged. A charging is effected only again when the voltage at the capacitor has decreased to such an extent that U'A has become smaller than UR. Then there is effected the switching-through of the operational amplifier into the output condition low and the switching- through of the transistor 24 at the switching flank of the sine half-wave in the aforedescribed manner. The circuit works by reason of the aforedescribed factors in a very delicately stepped and very sensitive manner and stabilises in low-loss manner flunctuations of the input voltage and fluctuations of the output voltage at the capacitor. Through the switching at both flanks of a sine half-wave in the vicinity of the zero passage, the arrangement also has a high efficiency.

Shown in FIGURE 6 is a second exemplified embodiment of the invention, in which a Schmitt trigger 50 is provided instead of the operational amplifier 27. All the remaining components bear the reference numbers in accordance with FIGURE 5. The Schmitt trigger is shown only as a block wiring diagram; a commercially available 12 IC can be used f or this. The inbuilt threshold voltage of the Schmitt trigger is used as the reference voltage UR The Schmitt trigger is provided with the required switching-hysteresis. The supply of the Schmitt trigger. with an operating voltage can be effected in the customary manner, also a voltage supply similar to that from resistor 28 and sub-circuit 35 of FIGURE 5 can be effected. The mode of operation of this arrangement emerges by reason of the detailed description of FIGURE 5 taking into account the known inherent threshold-value automatic behaviour of Schmitt triggers.

13 1

Claims

1. A method of generating a stabilised low voltage from a considerably higher pulsating direct voltage, for operating an electrical load, characterised in that an output capacitor is charged in accordance with the energy flow-off to the load, in that the energy supply to the capacitor is effected by switching of a switching element in the circuit of the capacitor respectively at both flanks of each sine half-wave or like pulse of the pulsating direct voltage supply close to the zero or low point between successive pulses, in that the switching element is switched on and off by a comparator arrangement which effects an evaluation, in conjunction with a combination, of voltage variations at the capacitor and voltage variations of the pulsating direct voltage together with a reference voltage.

2. A method according to Claim 1, characterised in that the comparison is effected with switching-hysteresis.

3. A method according to Claim 1 or 2, characterised in that at the one input of the comparator the voltage variations at the capacitor and the variations of the pulsating direct voltage are linked together and are compared in the comparator with the reference voltage.

4. A method according to Claim 2, 2 or 3, characterised in that the voltage at the output capacitor is smaller.by at least a factor of five than the pulsating direct voltage at the input.

5. A method of generating a stabilised low voltage output from a higher pulsatory direct voltage supply, for supplying a load from an output capacitor, wherein the capacitor is charged by switching on the supply to the capacitor during periods in which the pulsating voltage 14 is falling to and rising from zero or a minimum, and wherein said switching is controlled by comparative evaluation of the supply voltage, the output voltage and a reference voltage to effect switching on only when both said supply and output voltages are below predetermined limits.

6. A method as claimed in Claim 5 wherein the comparison is made between the reference voltage and a direct electrical combination of voltages derived from the supply and output voltages.

7. A method as claimed in Claim 5 wherein the comparison is made between a voltage derived from the output voltage and an electronic-logic combination of the reference voltage and a voltage derived from the supply voltage.

8. A method as claimed in Claim 5 wherein the comparison is made in stages firstly between the reference voltage and a voltage derived from the output voltage, and secondly between the result thereof and a voltage derived from the supply voltage, by further logic.

9. A method of generating a stabilised low-voltage output substantially as hereinbefore described with reference to FIGURE 1, FIGURES 1 to 4, FIGURE 5 or FIGURE 6 of the accompanying drawings.

10. Apparatus for supplying a stabilised low voltage output from a pulsatory direct voltage supply for carrying out the method comprising a switching element, such as a switching transistor, arranged between one of the input terminals and one of the output terminals; an output capacitor lying in parallel with the output terminals, which capacitor is chargable by the switching is element in accordance with its discharge in to load by switching current surges generated from both flanks of each input sine half-wave or like pulse; and comparator arrangement provided with or providing 5 switching-hystersis and a reference voltage (UR), One. input of which comparator arrangement receives a voltage (U? A) derived from the voltage at the capacitor; wherein a voltage (U'E) derived from the pu lsating direct voltage supply is linked electrically or logically with said one- input, the reference voltage or an output of the comparator arrangement to give a switch control signal; and wherein the switch control signal output of the comparator arrangement is connected to a control input of the switching element so that the latter switches through or respectively blocks said surges upon reaching of predetermined comparative conditions of said voltages (URMU'E) and (UIA).

a

11. Apparatus as claimed in Claim 10, wherein the switching element comprises a f irst transistor which is arranged in series with said ones of the input and output terminals and which is switched by a second transistor arranged in a shunt circuit and controlled by the switch control signal.

12. Apparatus as claimed in Claim 10 or 11, wherein the comparator arrangement comprises an operational amplifier.

13. Apparatus as claimed in Claim 10 or 11, wherein the comparator arrangement comprises a Schmitt trigger.

14. Apparatus as claimed in Claim 10, 11 or 12, wherein at the one input of the comparator arrangement the voltage (UIA) derived from the capacitor voltage and the 16 voltage (U'E) derived f rom the pulsating direct voltage are applied, and at the other input the reference voltage (UR) is applied.

15. Apparatus as claimed in Claim 14. wherein the reference voltage (UR) arises at a Zener diode lying in series with a resistor between the input terminals, and wherein a capacitor is connected in parallel with the Zener diode.

16. Apparatus as claimed in any one of Claims 10 to 15 wherein a voltage divider is arranged in parallel with the output capacitor to provide the derived voltage (U'A) for the comparator.

17. Apparatus as claimed in any one of Claims 10 to 16 wherein a voltage divider is arranged between the input terminals to provide the voltage (U'E) for the comparator arrangement.

18. Apparatus as claimed in any one of Claims 10 to 17 arranged so that said surges have a maximum voltage of less than three times, and preferably a maximum voltage of about 2.8 times, the predetermined output voltage, which surges are conducted via a limiting impedance and the switching element to the output capacitor.

19. Apparatus constructed and arranged to perform the method claimed in any one of Claims 1 to 9.

20. Apparatus substantially as hereinbefore described with reference to FIGURE 1, FIGURE 5 or FIGURE 6 of the accompanying drawings.

Published 1988 at The Patent Office, State House, 6671 High Holborn. London WC1R 4TP. Further copies maybe obtained from The Patent 0Mce, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray. Kent. Con- 1187,